Blood is the circulating tissue of an organism that carries oxygen and nutritive materials to the tissues and removes carbon dioxide and various metabolic products for excretion. Whole blood consists of a pale yellow or gray yellow fluid, plasma, in which are suspended red blood cells, white blood cells, and platelets.
An accurate measurement of hemostasis, i.e., the ability of a patient's blood to coagulate and dissolve, in a timely and effective fashion is crucial to certain surgical and medical procedures. Accelerated (rapid) and accurate detection of abnormal hemostasis is also of particular importance in respect of appropriate treatment to be given to patients suffering from hemostasis disorders and to whom it may be necessary to administer anti-coagulants, antifibrinolytic agents, thrombolytic agents, anti-platelet agents, or blood components in a quantity which must clearly be determined after taking into account the abnormal components, cells or “factors” of the patient's blood which may be contributing to the hemostasis disorder.
Hemostasis is a dynamic, extremely complex process involving many interacting factors, which include coagulation and fibrinolytic proteins, activators, inhibitors and cellular elements, such as platelet cytoskeleton, platelet cytoplasmic granules and platelet cell surfaces. As a result, during activation, no factor remains static or works in isolation. Thus, to be complete, it is necessary to measure continuously all phases of patient hemostasis as a net product of whole blood components in a non-isolated, or static fashion. To give an example of the consequences of the measuring of an isolated part of hemostasis, assume that a patient developed fibrinolysis, which is caused by the activation of plasminogen into plasmin, an enzyme that breaks down the clot. In this scenario, a byproduct of this process of fibrinogen degrading product (FDP), which behaves as an anticoagulant. If the patient is tested only for anticoagulation and is treated accordingly, this patient may remain at risk due to not being treated with antifibrinolytic agents.
The end result of the hemostasis process is a three-dimensional network of polymerized fibrin(ogen) fibers which together with platelet glycoprotein IIb/IIIa (GPIIb/IIIa) receptor bonding forms the final clot (FIG. 1). A unique property of this network structure is that it behaves as a rigid elastic solid, capable of resisting deforming shear stress of the circulating blood. The strength of the final clot to resist deforming shear stress is determined by the structure and density of the fibrin fiber network and by the forces exerted by the participating platelets.
Platelets have been shown to effect the mechanical strength of fibrin in at least two ways. First, by acting as node branching points, they significantly enhance fibrin structure rigidity. Secondly, by exerting a “tugging” force on fibers, by the contractability of platelet actomyosin, a muscle protein that is a part of a cytoskeleton-mediated contractibility apparatus. The force of this contractability further enhances the strength of the fibrin structure. The platelet receptor GPIIb/IIIa appears crucial in anchoring polymerizing fibers to the underlying cytoskeleton contractile apparatus in activated platelets, thereby mediating the transfer of mechanical force.
Thus, the clot that develops and adheres to the damaged vascular system as a result of activated hemostasis and resists the deforming shear stress of the circulating blood is, in essence a mechanical device, formed to provide a “temporary stopper,” that resists the shear force of circulating blood during vascular recovery. The kinetics, strength, and stability of the clot, that is its physical property to resist the deforming shear force of the circulating blood, determine its capacity to do the work of hemostasis, which is to stop hemorrhage without permitting inappropriate thrombosis. This is exactly what the Thrombelastograph® (TEG®) system, described below, was designed to measure, which is the time it takes for initial fibrin formation, the time it takes for the clot to reach its maximum strength, the actual maximum strength, and the clot's stability.
Hemostasis analyzer instruments have been known since Professor Helmut Hartert developed such a device in Germany in the 1940's. One type of hemostasis analyzer is described in commonly assigned U.S. Pat. No. 5,223,227, the disclosure of which is hereby expressly incorporated herein by reference. This instrument, the TEG® hemostasis analyzer, monitors the elastic properties of blood as it is induced to clot under a low shear environment resembling sluggish venous blood flow. The patterns of changes in shear elasticity of the developing clot enable the determination of the kinetics of clot formation, as well as the strength and stability of the formed clot; in short, the mechanical properties of the developing clot. As described above, the kinetics, strength and stability of the clot provides information about the ability of the clot to perform “mechanical work,” i.e., resisting the deforming shear stress of the circulating blood; in essence, the clot is the elementary machine of hemostasis, and the TEG® analyzer measures the ability of the clot to perform mechanical work throughout its structural development. The TEG® system measures continuously all phases of patient hemostasis as a net product of whole blood components in a non-isolated, or static fashion from the time of test initiation until initial fibrin formation, through clot rate strengthening and ultimately clot strength through fibrin platelet bonding via platelet GPIIb/IIIa receptors and clot lysis.
Platelets play a critical role in mediating ischemic complications in prothrombotic (thrombophilic) patients. The use of GPIIb/IIIa inhibitor agents in thrombophilic patients or as an adjunct to percutaneous coronary angioplasty (PTCA) is rapidly becoming the standard of care. Inhibition of the GPIIb/IIIa receptor is an extremely potent form of antiplatelet therapy that can result in reduction of risk of death and myocardial infarction, but can also result in a dramatic risk of hemorrhage. The reason for the potential of bleeding or non-attainment of adequate therapeutic level of platelet inhibition is the weight-adjusted platelet blocker treatment algorithm that is used in spite of the fact that there is considerable person-to-person variability. This is an issue in part due to differences in platelet count and variability in the number of GPIIb/IIIa receptors per platelet and their ligand binding functions.
Since the clinical introduction of the murine/human chimeric antibody fragment c7E3 Fab (abciximab, ReoPro®), several synthetic forms of GPIIb/IIIa antagonists have also been approved such as Aggrastatg® (tirofiban) and Integrilin® (eptifibatide), resulting in widespread and increasing use of GPIIb/IIIa inhibitor therapy in interventional cardiology procedures.
Before the introduction of the method and apparatus described in the afore-mentioned U.S. Pat. No. 6,613,573, there was no rapid, reliable, quantitative, point-of-care test for monitoring therapeutic platelet blockade. Although the turbidimetric aggregation test has been used to measure the degree of platelet GPIIb/IIIa receptor blockade in small clinical studies and dose-finding studies, its routine clinical use for dosing GPIIb/IIIa receptor antagonists in individual patients has not been feasible. Aggregation is time-consuming (more than one hour), expensive to run, requires specialized personnel for its performance, and is not readily available around the clock; therefore it cannot be employed at the point-of-care for routine patient monitoring and dose individualization. To be clinically useful, an assay of platelet inhibition must provide rapid and reliable information regarding receptor blockade at the bedside thereby permitting dose modification to achieve the desired anti-platelet effect.
The turbidimetric aggregation test is based on the photometric principle, which monitors the change in the specimen's optical density. Initially, a minimal amount of light passes through the specimen, as functional platelets are activated by the turbidimetric test; platelet aggregation occurs via platelet GPIIb/IIIa receptor and fibrin(ogen) bonding as illustrated in FIG. 1, and thus light transmission increases. When platelets are inhibited through GPIIb/IIIa receptor blockade, light transmission increases proportionally.
Another commercially available system measures fibrinogen-platelet bonding using beads coated with a fixed amount of an outside source of “normal” fibrinogen. Therefore, this system uses a non-patient source of “normal” fibrinogen and cannot detect a patient in a prothrombotic state (hypercoagulable) due to a higher patient level of fibrinogen, or detect a hemorrhagic state (hypocoagulability) due to a low patient level of fibrinogen. Additionally, this system shows only bonding without detection of the breakdown of that bonding. Therefore, in the presence of thrombolysis, the assessment of platelet GPIIb/IIIa receptor blockade by the system may not be accurate.
Fibrinogen-platelet GPIIb/IIIa bonding is the initial phase of platelet aggregation, or a primary hemostasis platelet plug, which goes on to form the final fibrin-platelet bonding. Thus it is not sufficient to measure only the initial stage of fibrinogen-platelet bonding, which may not accurately reflect final fibrin-platelet bonding via the GPIIb/IIIa receptor. While the turbidimetric and other photometric systems do detect initiation of platelet aggregation via fibrinogen-platelet GPIIb/IIIa receptor bonding, it may not accurately reflect final fibrin-platelet bonding via the GPIIb/IIIa receptor.
Significant among the limitations of systems that use beads coated with “normal” fibrinogen is that this “normal” fibrinogen may not reflect either the quantity or the functionality of a specific patient's own fibrinogen. Therefore, fibrinogen-platelet GPIIb/IIIa receptor blockade as measured by such systems is but a rough estimate of the patient's individual fibrinogen-platelet GPIIb/IIIa blockade of the initial phase of platelet aggregation.
This is a significant limitation in certain high risk patient subgroups, which may need treatment with a platelet inhibition agent, may have a higher or lower level of fibrinogen and thus would need an accurate assessment of platelet GPIIb/IIIa receptor blockade to reduce bleeding complications due to under assessment of platelet GPIIb/IIIa receptor blockade, or ischemic events due to over assessment of platelet GPIIb/IIIa receptor blockade. In addition, fibrinogen level and functionality may change during the trauma of interventional procedures. At this time it is imperative to make an accurate assessment of platelet GPIIb/IIIa receptor blockade in real time, during and following the procedure.
Thus, there is a need for a method and apparatus for measuring the efficacy of anti-platelet agents, continuously and over the entire hemostasis process from initial clot formation through lysis.